1. Field of the Invention
The present invention relates to an original scanning apparatus for optically reading an original placed on an original table in accordance with the movement of an optical system. The present invention also relates to an image forming apparatus incorporating the original scanning apparatus.
2. Description of the Related Art
In general, an image forming apparatus, e.g., an electronic copying machine is provided with a scanner, i.e., an original scanning apparatus, for optically reading the image on an original placed on an original table. The scanner comprises an exposure lamp for emitting light illuminating the original. The scanner also comprises an optical system including mirrors which guide the light reflected by the original to a photosensitive drum. With the optical system moved along the lower side of the original table, the image on the original is read.
In the scanner incorporating a stepping motor so as to drive the optical system, the optical system is driven slowly when it is advanced for scanning or reading the image on the original, and is driven fast when it is returned to the initial position. When the optical system is being returned, large torque is required for the acceleration or deceleration of the optical system. Therefore, a constant current circuit is employed to drive the stepping motor. When the optical system is being advanced for scanning, small torque is sufficient. In this case, therefore, the constant current circuit is so set as to produce a small amount of current. Thus, the motor and the constant current circuit are prevented from producing much heat, to thereby enhance the efficiency of the scanner.
In a conventional image forming apparatus, the optical system of a scanner is quickly decelerated at the end of the returning movement so as to stop the optical system at the initial position. In some cases, the optical system has to be accelerated for the next scanning operation, immediately after it is stopped at the initial position. In such cases, the optical system is likely to vibrate. This vibration may adversely affect the image formation performed when the optical system is being accelerated. More specifically, the stepping motor for driving the optical system may become out of step, or its rotor and stator may shift in position with reference to each other, causing dislocation of the scanning optical system. As a result, an image produced by the image forming apparatus may be blurred.